# Square wave to sine wave conversion

Hello guys i am very new to this forum.
I am doing project in digital radio. can you tell me how to convert square wave into sine wave of frequency range 88 to 108 MHz.
Thanks in advance.pls suggest me a circuit

## Answers and Replies

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vk6kro
Perhaps you need to clarify this.

Do you already have a square wave signal that covers this range and you want to turn its output into a sinewave?

What are you using to do this?

What is the amplitude of the square wave?

You also need to clarify how pure a sinewave you need. In other words, how many dB below the carrier must the spurs be?

Will the carrier be modulated?

hello sir thank you for your reply.

I am modulating frequency of square wave by message signal through digital circuits. message signal is stream of bits. I need to convert this square wave into sine wave before transmission since my receiver circuit will accept only sine waves. The frequency of square wave that is to be modulated is in the range of 88-108MHz and its amplitude is 3.3v.

I assume your transmitted power will be low enough to fall under the FCC part 15 for unlicensed transmitters.

There are two ways I can think of to convert a square wave to a sine wave. The first is with repeated filtering. You may need a number high Q RF stages to accomplish this.

The second is with double integration. The first integration will give you a triangle wave and the second will give you something very close to a sine wave. This likely will be difficult to achieve at 88-108 MHz. It may be easier to do at a lower frequency and multiply the frequency up to 88-108 MHz.

vk6kro
At 100 MHz, the difference between a sine wave and a square wave is that the square wave contains harmonics on 300 MHz, 500 MHz, 700 MHz etc.

A receiver on 100 MHz will not be able to tell that there are harmonics on higher frequencies, however such signals will be able to cause interference with other services, such as TV reception.

So, you would need to use a low pass filter which would allow 100 MHz through, but not the higher frequencies.

You also need to know that transmission of any signal on the international FM band is illegal in most countries except for very low powered signals which cannot be heard outside your property.

Thank you so much. I am going go transmit within 1m area only with low power transmitter. Can you please suggest me any IC that contains multiplier with integrated local oscillator.

NascentOxygen
Staff Emeritus
IThe second is with double integration. The first integration will give you a triangle wave and the second will give you something very close to a sine wave.
That is going to introduce AM on top of the FM.

It might be feasible to synchronize a sinusoidal PLL to the square wave. Alas, I don't have a specific IC in mind.

That is going to introduce AM on top of the FM.
True, but only at an amount proportional to 150 kHz/100 MHz = 0.15% or -56 dBc, an amount which is easily handled by the limiter in an FM receiver.

vk6kro
Thank you so much. I am going go transmit within 1m area only with low power transmitter. Can you please suggest me any IC that contains multiplier with integrated local oscillator.
There are chips that divide by variable ratios and that also have VHF oscillators in them.
It is possible to make a PLL synthesizer with these, but you need some serious test equipment to do this.

More realistically, you can get small transmitters that take the output of a MP3 player and transmit it to the FM radio in a car. These are not too expensive and they have a digital frequency readout already.
So you can set it to be between FM stations and it is very stable, so it will stay there.

You could try here:
http://s.dealextreme.com/search/fm+transmitter.html?page=1

They even transmit in stereo.